How to point a compass arrow to a certain player?

This is actually possible with commands, but not for a long time.

You have to use the /execute command on the target player:

/execute <player> ~ ~ ~ spawnpoint <player> ~ ~ ~

<player> 1 is the one who sets the spawn whilst <player> 2 is you

The reason why it won't be so used properly is because of a few reasons:

1. You will need the chunk where the command block to be loaded at all times. So if any of you go too far, the repeating command block won't work. It doesn't really help if you put multiple repeating command blocks around the world just so you can get him to set the world spawn.
2. It's a disadvantage to the runner if we want to reconsider Reason 1, because the hunter can easily respawn at the runner's current position which makes it easier to kill them.

This is easily possible with commands. Do:

/execute @p ~ ~ ~ setworldspawn

This will make the compass point on them but unfortunately also makes players spawn there.